This invention relates generally to data communications and more specifically to a method and system for synchronization and clock recovery.
CDMA communication systems rely heavily on the accuracy of PN code acquisition and tracking loops to establish communication. In the absence of phase, frequency, and timing information, any PN code acquisition system must employ a non-coherent strategy to achieve PN code synchronization at the receiver. Typically, upon acquiring the initial epoch of the received PN code, the tracking phase of PN code synchronization is initiated. Historically, this phase of the code synchronization has been accomplished using a non-coherent mechanism that is insensitive to phase, frequency, as well as to the modulation that is typically imposed on the received PN code [See M. K. Simon, J. K. Omura, R. A. Scholtz, and B. K. Levitt. Spread Spectrum Communications, volume III. Computer Science Press, Rockville, Md. (hereinafter referred to as xe2x80x9cSimon, et al.xe2x80x9d)], although a coherent version of PN code tracker has also been investigated. In a recent study, a decision-directed coherent DLL was proposed and investigated [See R. D. Gaudenzi and M. Luise. Decision-directed coherent delay-lock tracking for ds-spread-spectrum signals. IEEE Transactions on Communications, 39:758-765, May, 1991 (hereinafter referred to as xe2x80x9cGaudenzi, et al.xe2x80x9d)]. It was, subsequently, demonstrated that the overall loop performance is superior to a non-data-aided DLL. However, to the best of the authors"" knowledge, a channel-aided, decision-directed DLL has never been investigated when multi-user interference, frequency-selective Rayleigh fading, and log-normal shadowing are present. It is important to note that this study is different from its predecessor in a number of ways. First, a general M-ary phase shift keying (MPSK) modulation is considered, whereas binary PSK signaling is analyzed in Gaudenzi, et al. Second, we consider a complex PN spreading scenario here. Third, we consider a scenario wherein, in addition to phase synchronization, an amplitude estimation and matching is performed. This action will enhance the performance of the tracking loop when the desired signal that we intend to track is impaired by fading, noise, or interference. Finally, the analysis in Gaudenzi, et al. considers either an non-return-to-zero (NRZ) or a bi-xcfx86 chip pulse shape, whereas we consider the commonly used square-root raised cosine pulse shaping here.
The motivation for this invention stems from the fact that several proposals for future CDMA wireless systems have suggested a pilot-symbol-aided (PSA) mechanism for channel estimation. This, in turn, allows for a coherent reception in the face of fast channel fading in the uplink as well as in the down link of a wireless system. Since data detection is performed in the presence of channel estimation, one can use the combined channel and data information to realize a channel-aided, decision-directed delay-locked loop (CADD-DLL). More important, in a wireless CDMA environment, the received signal is corrupted by frequency-selective Rayleigh fading, log-normal shadowing, and user-induced interference, and hence a meaningful analysis must consider these impairments into account. The frequency selective Rayleigh fading in a CDMA environment results in the presence of a number of signal components which appear as the delayed versions of the original signal with random (complex) amplitudes. The presence of such complex multiplicative distortions (MD), which may be modeled as independent, bandlimited complex Gaussian processes, hampers coherent communication when the Doppler rate is comparable to the desired coherence interval (which is typically the symbol duration) of the signal.
In this invention, then, we are concerned with the problem of PN code tracking when a PSA scenario is considered and when the demodulated data and channel MD estimates are used to remove modulation and to compensate for the channel MD so that a CADD-DLL PN code tracking system can be realized. We conjecture that, for a reasonable level of signal-to-noise ratio (SNR), the data errors are rather infrequent, leading to a PN code tracking performance that is superior to its conventional counterpart.
The present invention is a channel-aided, decision-directed delay-locked loop (CADD-DLL). One embodiment of the present invention is implemented for pilot-symbol-aided (PSA) code-division multiple-access (CDMA) communication. In one embodiment of the present invention initial pseudo-noise (PN) code acquisition is accomplished with the aid of a conventional non-coherent (signal not in frequency and phase alignment) PN code acquisition system, and, upon acquiring the initial PN code epoch (the code repeat time interval), PN code tracking is performed using a channel-aided, decision-directed PN code tracking mechanism. The tracking loop in accordance with the present invention includes delay and advance PN correlators. The correlators are followed by data and phase correction as well as amplitude matching devices, the outputs of which are subtracted to form an error signal for code tracking purposes. The performance of the proposed tracking loop is assessed in terms of the variance of PN code tracking error and the mean-time-to-loss (MTTL) for a frequency-selective Rayleigh fading channel in the presence of user-induced interference and log-normal shadowing. It is demonstrated that the impact of the decision-directed mechanism on the performance is to reduce the variance of timing error for moderate levels of signal-to-noise ratio as compared to the conventional non-coherent delay-locked loop (NC-DLL) tracking mechanism. Also, the MTTL is shown to be highly dependent on the mobile""s Doppler shift.
These and other features of the invention that will be apparent to those skilled in the art from the following detailed description of the invention, taken together with the accompanying drawings.